1. Field of the Invention
The present invention relates to a plastic sheet which has creasing lines, each formed of a groove having a bottom surface and a pair of opposed side surfaces each slanting at a certain angle, as well as to a creasing-line-forming blade for forming the plastic sheet.
When a packaging container is to be formed through bending a plastic sheet, as shown in FIG. 1, grooves 102 are formed on a sheet 1 punched into a planar shape corresponding to the shape of the container, and the sheet 1 is then bent along the grooves 102 to thereby complete the container as shown in FIG. 2. The grooves 102 are called xe2x80x9clines for foldingxe2x80x9d or simply xe2x80x9ccreasing lines.xe2x80x9d The creasing lines are formed by a process such that a member called a xe2x80x9ccreasing-line-forming bladexe2x80x9d is pressed against the sheet 1.
The technique for bending a sheet after formation of creasing lines by use of creasing-line-forming blade has conventionally been used for fabrication of paper containers. However, when this technique is applied to fabrication of plastic containers, fabrication of containers having corners of a desired angle is difficult, because a plastic sheet has higher resistance against bending and higher elasticity than does a paper sheet.
Various techniques for solving the above-described problems have been proposed. One solution is employment of creasing lines having a special shape to thereby facilitate bending operation. For example, Japanese Utility Model Publication (kokoku) No. 4-9345 discloses a plastic sheet having creasing lines each formed of a groove in which projections and depressions are formed alternately on the bottom surface along the longitudinal direction (along a creasing line) thereof. Japanese Patent Application Laid-Open (kokai) No. 64-40317 discloses a plastic sheet having creasing lines each formed of a groove in which holes are formed in the bottom portion at a predetermined pitch along the longitudinal direction thereof.
In relation to a method of bending a plastic sheet, Japanese Patent Application Laid-Open (kokai) No. 2-98422 discloses a method in which a plastic sheet having creasing lines is first folded at each of the creasing lines, then unfolded to the original state, and then subjected to a bending process.
Furthermore, a creasing-line-forming blade having an improved structure has been proposed. Japanese Patent Application Laid-Open (kokai) No. 1-141720 (Japanese Patent No. 2541252) discloses a creasing-line-forming blade for a plastic sheet whose tip end has depressed portions and projected portions arranged alternately along the longitudinal direction, wherein the depressed portion has a length of 0.3 to 2 mm, the projected portion has a length of 0.02 to 0.15 mm, the tip end of the projected portion has a width not greater than 0.5 mm, and the tip angle is 30 to 130xc2x0. FIG. 3 shows a perspective view of an example of such a creasing-line-forming blade. Each projected portion 210 of a creasing-line-forming blade 2 has a sharp point, and each depressed portion 211 of the creasing-line-forming blade 2 has a flat shape. However, the projected portion 210 is not necessarily required to have the shape of a sharp cutting edge.
FIG. 4 shows a plastic sheet 1 on which creasing lines 110 have been formed by use of the creasing-line-forming blade 2. Upon the creasing-line-forming blade 2 being pressed onto the plastic sheet 1, the plastic sheet 1 deforms, so that creasing lines 110 are formed. Almost no plastic material remains at each groove portion 111 formed by means of the projected portion 210 of the creasing-line-forming blade 2, but plastic material remains at each projection 112 formed by means of the depressed portion 211 of the creasing-line-forming blade 2.
When an automatic container fabrication machine is used, a punched plastic sheet having creasing lines can be automatically formed into a three-dimensional container and sealed after placement of an article therein. However, such automatic container fabrication machine has involved a problem in that the automatic container fabrication machine fails to form the plastic sheet into a three-dimensional structure due to breakage of a creased portion. Increasing the thickness of portions where plastic material remains is one measure for avoiding such a problem of breakage. However, in this case, bending the plastic sheet becomes difficult, along with forming the plastic sheet by use of an automatic container fabrication machine. Japanese Patent Application Laid-Open (kokai) No. 2001-62909 (Title of the Invention: PLASTIC SHEET HAVING CREASING LINES AND CREASING-LINE-FORMING BLADE FOR PLASTIC SHEET) discloses an invention which can cope with the above-discussed problems. This invention is directed to a creasing-line-forming blade for forming creasing lines which have a novel shape and hardly break, as well as to a plastic sheet which is formed by use of the creasing-line-forming blade and which can be formed into a container by use of an automatic container fabrication machine.
An automatic container fabrication machine can perform, at high speed, a series of operations for bending a sheet to complete a three-dimensional container, charging a liquid or the like into the container, and sealing the container. Although the above-described techniques have enabled fabrication of containers through bending of a plastic sheet having creasing lines, the conventional plastic sheet cannot completely cope with such an automatic container fabrication machine. Use of the conventional plastic sheet will raise problems such as breakage of a creasing line portion, and failure in formation of a three-dimensional shape.
In a plastic sheet having creasing lines, bending operation is facilitated through a decrease in the thickness (residual thickness) of the plastic sheet at the bottom of each groove serving as a creasing line. However, when the residual thickness is decreased, a strong force tends to act locally at creasing line portions during bending, especially during a step of forming a plastic sheet into a final shape by use of an automatic container fabrication machine, thereby resulting in breakage of the container from a creasing line portion. This problem of breakage becomes remarkable when holes are provided at creasing line portions in order to facilitate a bending operation.
Although the invention disclosed in Japanese Patent Application Laid-Open No. 2001-62909 has solved the above-described problems to a certain level, further improved plastic sheets having creasing lines have been demanded.
An object of the present invention is to provide an improved plastic sheet which has creasing lines which are sufficiently soft to enable use of an automatic container fabrication machine and which can impart improved texture (sensation imparted when the creasing lines are touched).
Another object of the present invention is to provide a creasing-line-forming blade which is used for forming creasing lines on a plastic sheet to thereby produce the improved plastic sheet of the present invention.
The present invention provides a plastic sheet which has a plurality of creasing lines. Each creasing line comprises a main groove having a narrow bottom surface and a pair of opposed side surfaces each slanting at a certain angle; and a plurality of slant grooves disposed at a certain pitch in the longitudinal direction of the main groove. Each slant groove is formed by means of two or more curved projections to have at least one narrow bottom surface extending along an inclined direction with respect to the longitudinal direction of the main groove.
Preferably, the bottom surface of each slant groove extends at an angle of 20xc2x0 to 70xc2x0 with respect to a line perpendicular to the main groove. However, the angle may be changed in accordance with, for example, the material and thickness of the plastic sheet.
The present invention further provides a creasing-line-forming blade for plastic sheet. The blade has a narrow tip end extending in the width direction, and a pair of opposed side surfaces each continuing from the tip-end surface and slanting at a certain angle. A plurality of recesses are formed at the tip end at a certain pitch in the width direction. Each recess includes at least two curved depressions which are located adjacent to each other and extend along an inclined direction with respect to the width direction. The boundary portion between the curved depressions forms an inclined blade portion.
Preferably, the inclined blade portion extends at an angle of 20xc2x0 to 70xc2x0 with respect to a line perpendicular to the longitudinal blade portion. However, the angle may be changed in accordance with, for example, the material and thickness of the plastic sheet.